In some communication systems, a known pattern of symbols is used to indicate the beginning of a block of transmitted information, such as a transmitted packet. This may facilitate detection of the block, establishing symbol synchronization, etc. In present wireless local area network (WLAN) systems, for example, transmitters transmit packets having preambles. These preambles may include a known pattern of symbols. In such systems, a receiver may monitor the received signal in order to detect packets, and, in particular, may process the received signal to detect the preamble of a packet which indicates the beginning of the packet.
As an example, the Institute for Electrical and Electronics Engineers (IEEE) 802.11b wireless local area network (WLAN) standard specifies a protocol for WLAN systems and includes the use of patterns of symbols (e.g., preambles) to facilitate signal detection. According to the IEEE 802.11b WLAN standard, data is transmitted in packets, and the format of such packets is illustrated in FIGS. 1A and 1B. FIG. 1A is an illustration of an IEEE 802.11b packet 50 in a long preamble format. The packet 50 includes a preamble field 52, a header field 54, and a data field 56. The preamble field 52 includes a synchronization (SYNC) field 58 and a start frame delimiter (SFD) field 60. The SYNC field 58 includes a sequence of scrambled 1's and the SFD field 60 includes a known pattern of 1's and 0's. Each symbol in the SYNC field 58 and the SFD field 60 is spread by a length-11 Barker code sequence. FIG. 1B is an illustration of an IEEE 802.11b packet 70 in a short preamble format. The packet 70 includes a preamble field 72, a header field 74, and a data field 76. The preamble field 72 includes a SYNC field 78 and an SFD field 80. The SYNC field 78 includes a sequence of scrambled 0's, and the SFD field 80 includes a known pattern of 1's and 0's. Each symbol in the SYNC field 78 and the SFD field 80 is spread by the length-11 Barker code sequence.
At a receiver, the received signal may be cross-correlated with the Barker code sequence in order to detect preamble fields. As a received preamble is cross-correlated with the Barker code sequence, a peak will occur when the Barker code and a symbol of the preamble field overlap. Detection such peaks are used to detect the preamble generate the synchronization and timing information. When no signal is present or when the signal-to-noise level is poor, no peak or only small peaks may occur. A relative height of a peak generally is a peak-to-average measure of the cross-correlation signal.
A prior art apparatus 100 for generating synchronization information for a received signal that was transmitted according to the IEEE 802.11b protocol is illustrated in FIG. 2. The apparatus 100 generates a signal quality (SQ) signal, which is generally based on a cross-correlation of the Barker code sequence with the received signal. The apparatus 100 includes an analog-to-digital converter (ADC) 102 that converts an analog, down-converted I/Q signal to a digital I/Q signal at a sampling rate of 22 MHz. A Barker correlator 104 receives the digital I/Q signal and cross-correlates it with the Barker code sequence. FIG. 3 is a graph of an example output of the Barker correlator 104. In particular, FIG. 3 illustrates an I component correlation output corresponding to a received signal with a 1 MHz symbol rate. As can be seen in FIG. 3, a peak occurs approximately every 22 samples (which corresponds to 1 MHz).
Referring again to FIG. 2, a magnitude calculator 106 receives the output of the Barker correlator 104 and generates a magnitude signal. In particular, the magnitude calculator 106 generates the magnitude signal by adding the magnitude of the I component to the magnitude of the Q component. Then, a peak-to-average calculator 108 generates a peak-to-average signal, which generally provides a measure of the relative heights of peaks in the magnitude signal. The peak-to-average signal may be referred to as a signal quality (SQ) signal, and may be generated as:
                              SQ          =                                                    max                i                            ⁢                              (                                                                                                I                      i                                                                            +                                                                                Q                      i                                                                                          )                                                                    1                16                            ⁢                              (                                                                            ∑                      i                                        ⁢                                                                                        I                        i                                                                                                    +                                                                                Q                      i                                                                            -                                      2                    ⁢                                                                  max                        i                                            ⁢                                              (                                                                                                                                        I                              i                                                                                                            +                                                                                                                Q                              i                                                                                                                                  )                                                                                            )                                                    ,                            (                              Equ            .                                                  ⁢            1                    ⁢          a                )                                          SQ          =                                                    max                i                            ⁢                              (                                  magnitude                  i                                )                                                                    1                16                            ⁢                              (                                                                            ∑                      i                                        ⁢                                          magnitude                      i                                                        -                                      2                    ⁢                                                                  max                        i                                            ⁢                                              (                                                  magnitude                          i                                                )                                                                                            )                                                    ,                            (                              Equ            .                                                  ⁢            1                    ⁢          b                )            where i is an index from 0 to 21, and magnitude is the output of the magnitude calculator 106. Next, the SQ signal is compared to a threshold by a comparator 110. When the SQ signal exceeds the threshold, this may indicate the presence of a symbol and provide timing information regarding the symbol. SQ signal also can be used for Clear Channel Assessment (CCA).